Circuit arrangements of the type here considered are frequently used for the selective switching of a load to any of a large number of transformer taps, e.g. as shown in German laid-open application No. 2,120,679 published Nov. 18, 1971. Such a system comprises two mutually identical circuit branches having a common output connected to the load, the inputs of these branches being respectively connected at the time of switchover to a currently active source and a source about to be activated in its stead. When switchover is completed, the input of the now de-energized branch can be transferred to a different source to be next connected across the load. Thus, the load voltage can be selectively increased or reduced by each switching operation.
The system known from the above-identified German publication comprises in each branch a main switch, closed when that branch supplies the load current, and a pair of antiparallel controlled rectifiers or thyristors forming part of a collateral network which shunts the main switch and includes an ancillary switch in series with the thyristors. During steady-state operation, the thyristors of both networks are cut off; their gates are connected to a control unit which receives operating voltage from either of two power supplies that are engerized only when the ancillary switch of a respective circuit branch is closed while the associated main switch is open. This condition occurs only at a stage of beginning activation of that branch. The control unit then applies voltage to the gates of both thyristor pairs, enabling first a thyristor of the branch undergoing deactivation and next a thyristor of the branch about to be activated to fire during two consecutive half-cycles of the transformer voltage. Conduction of the first thyristor is initiated by the opening of the main switch of the branch being deactivated; closure of the other main switch must be timed to occur about one cycle later, i.e. upon termination of conduction of the second thyristor, since the control unit does not allow any thyristor to be fired more than once.
The use of two pairs of antiparallel thyristors diminishes arcing at the main switches during the changeover from one source to another, yet the aforedescribed system is complex and requires thyristors able to be rapidly fired and quenched.